The present application generally relates to treatment of objects. More particularly, the present invention provides an improved method for processing substrates with ultraviolet light in a single substrate wet processing tool. The invention also provides for improved processing of substrates with a vapor.
In a wet chemistry process a substrate is immersed in or otherwise treated with a liquid chemical solution. Examples of wet process steps include etching, photoresist stripping, removal of oxides with acids such as hydrofluoric acid (HF) and cleaning. Substrates that are processed in this manner include semiconductor wafers, hard disk media, stepper masks, and flat panels. It is often important that cleaning and etching processes do not damage the substrate, the process apparatus, or features formed on the substrate. Also, the etching apparatus is desirably easily scaled to process many substrate sizes, including large area wafers that are 300 millimeters or more in diameter.
To process wafers efficiently, single substrate tools have been developed that allow simultaneous processing of single substrates in multiple chambers that implement different steps of the process. Such single substrate tools generally reduce the time that any given chamber remains idle. The equipment conventionally used for wet processing substrates generally includes a series of tanks or sinks containing the chemical solution into which racks of semiconductor wafers are dipped. Since the tanks are typically open to the atmosphere, airborne particulates can enter into the process solutions. Through surface tension these particles are easily transferred to the wafer surfaces as the wafers are dipped into and lifted out of the sinks. This particulate contamination is detrimental to the microscopic circuits that the wafer fabrication process creates.
To reduce particulate contamination, some wet processes use ultrasonic energy to assist wafer cleaning and drying. In an ultrasonic wafer cleaning process, for example, a non-reactive vessel or container is filled with an acidic cleaning solution such as sulfuric acid/hydrogen peroxide or hydrochloric acid/hydrogen peroxide, and the solution is ultrasonically vibrated to remove contamination from a wafer surface. Such an ultrasonic cleaning process is described in detail in U.S. Pat. No. 5,505,785 assigned to Gary W. Ferrell. The ultrasonic vibration is believed to cause an acoustic streaming of the cleaning solution, which induces a microflow of the solvent across the surface of the wafer. This induced microflow provides for quick and efficient cleaning of the wafer at reduced temperatures.
In an ultrasonic drying process wafers are dried by a mist of ultrafine droplets. Such an ultrasonic drying process is described in detail in U.S. Pat. No. 5,653,045 assigned to Gary W. Ferrell. Typically, the wafers are immersed in de-ionized water as nitrogen-pressurized isopropyl alcohol (IPA) flows to a sonic head attached to the top of the drying chamber. The sonic head vibrates at about 100 kilohertz to produce a mist or fog of ultrafine IPA droplets having an average diameter of about 20 microns. A low-velocity gas nozzle coupled to the sonic head distributes the droplets into the drying chamber. The IPA mist settles on the surface of the de-ionized water and forms a film that partially diffuses into the water. The de-ionized water is then slowly drained from the drying chamber so that the low surface tension of the IPA film forces the de-ionized water from the surface of the wafers in a uniform sheet. After the de-ionized water has been completely drained from the drying chamber, hot nitrogen is introduced into the drying chamber to ensure drying of the wafers. Unfortunately, the slow rate at which the water is drained limits produces bottlenecks in single wafer processing tools.
Surface contaminants frequently comprise organic molecules. One technique for removing these contaminants from semiconductor wafers uses ultraviolet (UV) light to provide energy for chemical reactions such as wafer cleaning and oxide formation. Although it has long been known that ultraviolet (UV) light decomposes organic molecules, only in the last few years has UV cleaning of surfaces been explored. One such cleaning scheme is described in U.S. Pat. No. 5,814,156, assigned to UVTech Systems, Inc. In this scheme, a surface, contaminated by adsorbed hydrocarbons, is irradiated with UV light in an atmosphere of a reactive gas such as oxygen. The contaminants absorb the UV radiation and become excited. At low energy levels the bonds between the contaminant molecules and the surface tend to break and an inert gas can be used to carry away the debris. At higher energy levels some contaminant molecules tend to dissociate from the surface into free radicals that react with atomic oxygen, produced by the dissociation of ozone, to form carbon dioxide, water and nitrogen, while other contaminants tend to heat and expand quickly away from the surface.
If the UV light is produced outside the vacuum chamber, a window, typically made of quartz, couples the UV into the chamber. The window must be made thick since it must withstand a pressure difference on the order of 1 atmosphere (760 torr). Generally, the thicker the window the greater the attenuation of the UV.
In another cleaning scheme, a surface is irradiated in ambient air by laser pulses aimed normal to the surface to produce a spot size of several millimeters. Surface contaminants absorb the UV laser pulses, expand from highly localized heating, and accelerate away from the surface. The resulting particles are carried away with water or inert gas. Because this scheme takes place in ambient air, particulate contamination is a potential problem.
Dry processing in a plasma can generate a large amount of energetic UV photons via plasma afterglow emission. Such a plasma UV processing scheme is described in U.S. Pat. No. 5,217,559 assigned to Texas Instruments. These UV photons are produced in situ and can be used for photochemical processing with minimal attenuation. Unfortunately, plasma processing techniques require a large amount of process gases. Typically, only a small percentage of these process gases participate in useful chemical reactions. Hence, a large portion of the process gases is wasted. Even worse, the process gases can actually etch and damage the reaction chamber containing the plasma, necessitating frequent maintenance. Furthermore, the substrate that is being processed is often exposed to the high temperature and energetic ions of the plasma. Semiconductor devices with fine geometry features can easily be damaged by the temperatures and ion bombardment associated with plasma processing. Further, high temperatures severely limit the kinds of processes that are available. Substrates having metal structures, polyamide structures, or soft glass structures cannot be exposed to high temperatures.
An additional drawback of known single substrate processing tools is the mechanical pick-up attached to the robot arm. Ideally, the pick-up should minimally contact the surface of the substrate. The pick-up generally has one or more "fingers" that grip the edge of the substrate. This does not present a problem for small diameter substrates such as six-inch semiconductor wafers. However, as the substrate diameter becomes larger it becomes more difficult for the pick-up to grip the wafer without bending or dropping them. Furthermore, stress applied at the edge of a substrate containing multiple thin films can cause delamination of the films. Liquid also tends to collect at edge contact points.
Furthermore, today's 300-mm wafers are very susceptible to contamination on both the front and back sides. Such wafers are generally polished on both the front and back sides. In most plasma processing techniques, however, the wafer typically rests on or adheres to a substrate support and only one side of the wafer is exposed. It would be desirable to use UV in a single wafer tool wet process since damage to the wafer can be minimized, both sides of the wafer can be exposed to UV and particulate contamination can be controlled. Unfortunately, UV light is strongly attenuated by most liquids. UV light generally penetrates only a few millimeters into water based liquid solutions. Therefore, although UV processing is now common in vacuum, plasma, or dry gas substrate processing, UV light is not normally used in wet processing.
Therefore, a need exists in the art for a fast clean and efficient method and apparatus for processing an object using with ultraviolet light a single object tool wet processing system.